This application defines the mitochondrial quality control (QC) network in the lung as a therapeutic target during acute lung injury (ALI/acute respiratory distress syndrome; ARDS), a major critical illness caused by injury to the alveolar-capillary barrier. The proposal is based on recent developments from our laboratory on the role of mitochondria in the mechanisms of ALI resolution. ALI is caused by severe damage to the alveolar-capillary barrier, leading to an exudative phase of protein-rich edema and cellular debris resulting in physiological shunt and refractory hypoxemia. Severity is such that the abundant cuboidal alveolar type II (AT2) cell, the source of surfactant production/recycling, is also thought to be necessary to regenerate type I cell epithelium and restore epithelial integrity. Accordingly, if the proliferative phase is controlled, the resolution of alveolar barrier integrity and liquid clearance gradually occurs. The AT2 cell also supports various non-surfactant alveolar functions from sodium transport to host defense that require ATP, and hence are enriched in mitochondria. These mitochondria are maintained by a sentinel genetic mitochondrial QC network, which is also required for cell proliferation and possibly for alveolar regeneration. Our preliminary data shows that the transcriptional program that regulates the mitochondrial QC network becomes highly activated in the lung AT2 cell during experimental Staphylococcal aureus pneumonia/ALI in mice. The resolution of ALI depends on the inducible heme oxygenase-1 (HO-1; Hmox1) system in the AT2 cell and is regulated by and regulates antioxidant response element (ARE)-dependent genes. Specifically, the DNA-binding transcription factor NRF-1 (Nuclear Respiratory Factor-1), a partner of the PGC-1? co-activator, is critical not only to maintain bioenergetic function, but for the AT2 cell to serve as an AT1 cell progenitor. We propose that the AT2 cell mitochondrial QC program is necessary for AT1 proliferation and the full resolution of alveolar epithelial damage in ALI. This idea is not just novel; it is amenable to therapeutic intervention. Thus, targeted pharmacological interventions may shorten the course of ARDS. Therefore, using molecular methods with a pre- clinical approach in mice and cells, we will rigorously test transcriptional activation of mitochondrial QC in the AT2 cell as a critical factor in ALI resolution through enablement of AT2 cell proliferation, trans-differentiation into AT1 cells, and control of inflammation, all of which are necessary to restore epithelial barrier function. We will test our novel ideas with three Specific Aims: Aim 1) Measure and localize oxidant damage in lung parenchyma and AT2 cell mitochondria (specifically mtDNA oxidation) in ALI/pneumonia in mice and its impact on mitochondrial QC regulation by HO-1/CO and a) AT2 cell apoptosis, b) resolution of alveolar inflammation and c) alveolar barrier dysfunction. Aim 2) Test how HO-1 induction of the mitochondrial QC network a) activates mitophagy b) accelerates AT2 cell proliferation and trans- differentiation into type I epithelium, and c) prevents necroptosis after ALI/pneumonia. Aim 3) Demonstrate whether strategies that activate mitochondrial QC will reverse loss of lung protection in ALI/pneumonia in mice with conditional knockout (CKO) of a) HO-1 or b) NRF-1 in AT2 cells using the approaches of Aim 1.